I believe that E10 is mandated in the Chicago metropolitan area (and pure gas isn’t legal for sale), in the name of reducing air pollution. IIRC, the refineries used to use MTBE for this purpose, but were forced to switched away from it about 10 years ago due to health concerns around MTBE. And, this being a state where an awful lot of corn is grown, mandating ethanol in the Chicago area likely went over very well downstate.
Perhaps I misunderstood you. I took your use of the word “nowadays” to mean that a higher percentage of cars used to require octane fuel than now do. I’d also bet that the majority of cars are still normally aspirated, but I think that a more substantial minority than in the past require high-octane fuel.
Octane requirements are related to an engine’s compression ratio and yep, “economy” cars are coming with high compression engines(10.0:1 and up) nowadays. The real surprise is the turbos hi-comp are as well, because early on turbos ran lower compression than NA; the first 911 turbos ran 6.5:1 (compared to 8.5:1 for the regular 911)!
Yes, turbos, especially high-performance ones, often have a relatively low compression ratio, because that gives more control using variable boost. The effective CR on some engines can be double the mechanical one or more. This is another case where sensitive knock sensors are used to run the engine at the maximum boost and timing the fuel will permit. Running the same engine on 87 octane and 95 octane can mean a 200HP difference.